1. Technical Field
The present invention relates to stripline filters. More specifically, the present invention relates to a novel stripline tapped-line hairpin filter.
2. Background Art
Microstrip and stripline filters are employed for filtering microwave frequency signals or other types of high frequency signals. Microstrip and stripline filters are commonly used in high frequency filtering applications. One such application is in a radar system wherein received signals are filtered, i.e., signals of a particular frequency range pass through the filter, for further processing. Striplines have the inherent advantage over microstrip in that opposite surfaces of a substrate may have circuit elements disposed thereon. The stripline circuit element-substrate-circuit element structure is sandwiched between the two conductive groundplanes and insulated therefrom by two dielectric substrates. Microstrips typically have circuit elements formed on one surface of a dielectric substrate and a groundplane formed on the opposite surface.
One type of microwave frequency filter uses a microstrip parallel coupled filter. The microstrip parallel coupled filter has the disadvantage that input and output end sections are required to couple the signals into and out of the filter which is comprised of a plurality of N circuit elements. The input and output end sections are respectively parallel coupled to the first and last resonators of the N circuit element filter. Thus, additional surface area is required to form the additional input and output end sections for the N circuit element filter. A further disadvantage occurs in the situation where the parallel coupling at the end sections becomes very tight and physical realization becomes impractical.
Other types of microstrip filters include the tapped-line interdigital and combline filters. These type of microstrip filters have the advantage over the parallel coupled filters by virtue of the tapped-line feature. The tapped-line feature allows the first and last resonators to also serve as the input and output sections. This provides savings in space and an improvement in filter bandwidth. For example, a 20 to 30 percent bandwidth may be achieved using the tapped-line interdigital filter. However, physical limitations exist due to the coupling spacing requirements between adjacent microstrip filter elements, thus limiting further expansion of the filter bandwidth.
Another type of microstrip filter is the parallel coupled hairpin filter. The parallel coupled hairpin filter uses a plurality of N hairpin shaped resonators disposed on a surface of the substrate with alternating orientation. The parallel coupled hairpin filter requires input and output end sections which provide parallel coupling of the signal in and out of the filter. However, in certain situations the parallel coupling between the end sections and the first and last resonators may become very tight and physical realization may not be practical. Therefore, this type of filter is limited in bandwidth due to the tight coupling at the end sections. In addition, extra space is required for the end sections on the surface of the substrate.
Another type of microstrip filter is the tapped-line hairpin filter. The microstrip tapped-line hairpin filter eliminates the need for end sections to couple signals into and out of the parallel coupled hairpin resonators. This allows an increase of the bandwidth in the range of 30 to 40 percent. A design using the tapped-line hairpin filter is described in an article entitled "Microstrip Tapped-Line Filter Design" by Joseph S. Wong, IEEE Transactions On Microwave Theory and Techniques, Volume MTT-27, No. 1, January 1979.
In many applications the microstrip filter provides sufficient bandwidth. However, in some applications a greater bandwidth is required, such as in excess of 40 percent. Microstrip filters of this type will not permit bandwidths higher than 40 percent due to the physical limitations, i.e., required spacing between adjacent filter elements. Thus, the spacing requirement between adjacent microstrip filter elements limits the overall frequency bandwidth of the filter.
The microstrip approach limits the bandwidth due to the adjacent construction of the filter elements, on a single surface of the substrate. As the bandwidth increases the impedance between adjacent filter elements correspondingly increase, i.e., coupling becomes tighter. Since the tightest coupling occurs at the input and output adjacent resonators, once the coupling is too tight in these areas, the filter is no longer realizable.
It is, therefore, an object of the present invention to provide a wideband microwave filter.
It is another object of the present invention to provide a stripline bandpass filter for microwave applications having a wide bandwidth capability.
It is yet another object of the present invention to provide a stripline bandpass filter using hairpin resonators spaced alternately on opposite surfaces of a dielectric substrate wherein the first and last hairpin resonators are respectively tapped for signal input and output.